With the availability of microwave transistors and other semiconductor devices usable at microwave frequencies, the microstrip transmission line has found wide application because of its compatibility with the fabrication and installation of passive components and active devices on the same substrate with the transmission line. Essentially, a microstrip transmission line consists of a strip of conductive material, approximately corresponding to the center conductor of a coaxial transmission line, deposited on the top side of a dielectric or semiconductive substrate by photoresist techniques. The bottom side of the substrate is grounded in the area underneath the strip of conductive material on the top side and electrically corresponds to the outer cylindrical conductor of a coaxial transmission line.
Microstrip transmission lines, being open microwave structures, do not ordinarily exhibit loaded Q's much in excess of 500. Circuit losses due to dielectric dissipation, conductor resistance and leakage radiation confine Q values to approximately 1000 or less in the family of wave guide structures to which the microstrip belongs (e.g., triplate, coaxial, etc.).
There are numerous applications in microwave systems where high Q circuits and very low loss resonant ring transmission lines are highly desirable. The latter requirement arises generally with a concern for system noise and the former requirement for frequency stabilization, frequency identification, filtering, isolating, coupling, resonating and the like. Previous techniques for attaining high Q's in the neighborhood of about 1000 have involved the use of dielectric cavities of size compatible with the microstrip solid-state circuits and resonant microstrip structures of high conductivity metals (e.g., copper or gold) on the lowest loss dielectric substrate material available. Dielectric Q's are at present around 2000; and this would appear to limit the upper Q value obtainable with conventional microstrip circuit techniques.